The problem of monitoring liquid level in a reservoir has been addressed by prior art including Laycock U.S. Pat. No. 4,589,524. During pressure filling of the reservoir, the service operator frequently cannot see the reservoir because of the size of the equipment and the location of the reservoir. As overfilling as well as underfilling of the reservoir can be damaging, it is desirable for the service operator, while at a position remote from the reservoir, to be able to detect a properly filled condition and quickly discontinue the filling operation to prevent excessive overflow.